Curved mirrors are known as catoptrics and conventional lenses systems are known as dioptrics. Devices consisting of both are known as catadioptrics. Catadioptric systems can offer several advantages over conventional lens systems. Catadioptrics can be employed to shorten the overall optical path length as compared to standard dioptric lens stacks. This aspect makes catadioptrics popular in systems such as telescopes and in telescopic camera lenses since the length and weight of a catadioptric lens is significantly less than the length and weight of a corresponding dioptric lens system. Telephoto or telescopic catadioptric mirrors are typically concave and are rotationally symmetric around the optical axis of the lens.
Similarly, catadioptric systems can give a wide field of view by employing convex quadric mirrors. In catadioptric systems with rotationally symmetric convex quadric mirrors, panoramic images can be captured directly. Increasingly, these panoramic catadioptric systems are being employed for machine, robot and computer vision systems, including military applications. Specific mirror surface shapes are selected to generate related image attributes in some catadioptric systems. Further, software is generally employed to deconvolve acquired images from these types of catadioptric systems, e.g., converting a ‘circular image’ into a more conventional panoramic image.
Furthermore, in conventional dioptric lenses, the camera lens is focused at distances correlated to the depth of an object in an imaged scene. In contrast, catadioptric systems generally use a camera lens focused on a virtual object formed by reflection of an imaged scene object on a mirror. Where a rotationally symmetric quadric mirror is employed in the catadioptric system, the infinite range of scene depth is limited to a finite depth range of the virtual object points. The finite volume of the virtual object space is known as the caustic volume. Typically, a camera in this type of catadioptric system is placed such that the depth of field (DOF) captures the caustic volume. Thus, these cameras generally have high f-numbers (i.e., small apertures) and an increased distance between the imaging sensor and the virtual objects such that the DOF is sufficient to capture the majority of the caustic volume resulting in an image that is in focus. As these catadioptric systems evolve into even more compact systems or where larger apertures are needed (e.g., low light conditions), the DOF can become reduced so as not to capture the enough of the caustic volume to generate an overall well focused image. A shallow DOF can result in areas of the image being in focus while other areas are out of focus in catadioptric imaging.
The above-described deficiencies of conventional imaging are merely intended to provide a brief overview of some of the problems of conventional systems, and are not intended to be exhaustive. Other problems with conventional systems and corresponding benefits of the various non-limiting embodiments described herein may become further apparent upon review of the following description.